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insullation beneath floor with radiant heat
The more r-value you have below the less inflitration you get. Sounds like your floor r-value is around a 1.5. As long as my r-value below is higher then the 1.5 you will get heat transfer. Min insulation r-value for joist heating is a the least an R-11. If your using Wirsbo's joist trak plates you insullate directly to the plate. The reason other mfs have the spacing is that they need to create convection in the joist bay to get the btu output through the floor. With joist trak plates you are actually transfering the heat absorbed by the plate directly to the floor not creating an oven in the joist bay.
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."